A wide variety of medical and surgical procedures are optimally performed for best diagnostic or therapeutic efficacy when a surgical instrument is passed through tissue precisely to a target location. Common examples of these procedures include biopsy of suspected abnormal tissue or placement of a ventriculostomy or shunt catheter into the brain in the treatment of hydrocephalus. Techniques to allow improved guidance or positioning have been developed. However, these techniques all suffer from significant shortfalls.
Exemplifying these shortfalls are the techniques which may be used for the placement of a shunt catheter into the brain for treatment of hydrocephalus. Hydrocephalus is a disease principally of malabsorption of cerebrospinal fluid (CSF) that results in the gradual enlargement of the ventricles in the brain with effacement and eventual permanent injury to adjacent tissues. It is most commonly treated by drilling a hole in the skull and guiding a silicone shunt catheter or ventriculostomy tube into the ventricle. The cerebrospinal fluid is diverted from its normal and impeded path through the catheter. The shunt apparatus is usually further tunneled beneath the skin of scalp and body to terminate in the peritoneal cavity (VP shunt) or in the atrium of the heart (VA shunt) in order to direct the CSF to the body's circulatory system. The long-term success or control of hydrocephalus depends on proper positioning of the catheter. Misplacement of the catheter may lead to acute obstruction of the tube with brain tissue or delayed obstruction due to gradual ingrowth of choroid plexus. The obstructions then require that the catheter be replaced by additional surgery.
To minimize the risk of malposition, a surgeon has several options. For example, well-described, externally palpable landmarks of the cranium may be used to select and drill a hole. The catheter is then aimed toward another palpable landmark of the head, expecting to intersect an optimal target location at a depth judged from review of imaging studies such as computed tomography (CT) and magnetic resonance imaging (MRI). However, this technique is difficult to employ due to human variation in landmarks, variability in the ventricle size and position, and encumbrance of surgical drapes used to isolate a surgical field. Malposition attributable to these variables is a common complication.
Alternatively, real time ultrasound may be employed to guide a catheter precisely in infants through an open fontanel. However, the fontanel is normally closed after 18 months of age, forcing the creation of an otherwise unnecessary burr hole to image the brain at an older age.
A stereotaxic frame may also be employed to guide a catheter precisely, but this requires lengthy acquisition of target coordinates in a radiologic suite before surgery. The application of the frame is also an invasive procedure, since the frame is installed with a series of pins which are inserted into the cranium. Similarly, the more recent techniques of "frameless stereotaxy" require pre-operative localization of fiducials or markers for orientation and employ costly equipment for a simple procedure.
Intraoperative CT and MRI (so-called open MRI) allow precise guidance, but the necessary equipment for these procedures present significant hindrances to sterile setup, timely operation, operating room efficiency, and cost since such equipment is not ordinarily used in the operating room. These techniques also operate by emitting ionizing energy which may have cumulative potential injurious effect in the case of CT or interfering magnetic fields in the case of MRI.
Endoscopic adjunctive guidance of the shunt catheter facilitates final placement, but does not improve the trajectory accuracy through the soliditissue of the brain, the principal determinant of eventual position. Internal visual confirmation is also not normally necessary in the management of simple hydrocephalus.
A variety of external aligmnent devices have been developed. For example, the Ghajar guide, manufactured by Codman and Shurtleff of Raynham, Mass., is a guide tube which assures perpendicularity to the cranium. However, the Ghajar guide provides no depth control and requires a paramedian coronal entry burr hole. The guide also does not compensate for human variations of cranium or ventricle configuration and thus may result in malposition.
Thus, there exists a need for a catheter positioning device which: a) is simple, requiring minimal setup time, expense, training or expertise of the surgeon; b) allows precise guidance during surgery despite encumbrance of surgical drapes and difficulty at palpation of common landmarks; c) allows optimal target selection based on pre-operative imaging studies such as CT and MRI of the head; d) avoids invasiveness of a frame application to the patient or special imaging procedures to allow such guidance; and e) builds on the existing standards for the surgical approach to the ventricle system.
It is also desirable to have a guidance device which may be coupled to an endoscope or any other surgical instrument, allowing guidance to the target such that additional procedures such as biopsy or diagnostic ventriculoscopy may be performed with equal precision. A guidance technology which allows "on the fly" selection of alternative entry sites in the instance of recognized impediments in the path, e.g., a large vessel on the brain surface which should not be cut, is needed. There is also a need for a guidance device which allows a user to choose a minimal or a significantly curved trajectory toward a target rather than a straight line as forced by most of the aforementioned techniques. It would also be advantageous to rely on internal or within the body alignment instead of reference to external prominences, measured relationships, frames, line of sight or sound digitizers.